As long-life ball end mills capable of efficiently cutting high-hardness dies used for the production of various parts in automobile industries, electronic industries, etc., multi-flute ball end mills of cemented carbide having 3 or more ball-end cutting edges are widely used. In the cutting of a work by a ball end mill, however, a large load is applied to portions of ball-end cutting edges near a rotation center point, in which a rotation speed is substantially zero, thereby generating vibration. As a result, the ball-end cutting edges suffer chipping and breakage near the rotation center point. To solve this problem, various proposals have been made so far.
JP 2002-187011 A proposes, as shown in FIGS. 25 and 26, a multi-flute ball end mill having 3 or more ball-end cutting edges, in which a flank (land) of each ball-end cutting edge is thinned, and each ball-end cutting edge is notched near a rotation center point, to prevent an insufficient chip pocket near the rotation center point O. However, because a thinned portion of each ball-end cutting edge does not have an arcuate portion, a large load applied near the rotation center point O causes vibration. In addition, JP 2002-187011 A does not consider the radial rake angles and twist angles of ball-end cutting edges and peripheral cutting edges. Accordingly, when this multi-flute ball end mill is used for high-feed roughing of a high-hardness, difficult-to-cut material, chipping and breakage occur in the ball-end cutting edges and peripheral cutting edges.
JP 2009-56559 A proposes a ball end mill having 2 or more ball-end cutting edges, and grooves each having a V-shaped or U-shaped cross section and formed between the ball-end cutting edges near a rotation center point, thereby well discharging chips from a tool center portion even in high-efficiency cutting. However, because this ball end mill does not have cutting edges near the rotation center point, it suffers vibration due to a large load applied near the rotation center point. Further, JP 2009-56559 A does not consider the radial rake angles and twist angles of ball-end cutting edges and peripheral cutting edges. Accordingly, when this multi-flute ball end mill is used for high-feed roughing of a high-hardness, difficult-to-cut material, chipping and breakage occur in the ball-end cutting edges and peripheral cutting edges.
JP 9-267211 A discloses a two-flute ball end mill suitable for high-speed cutting of dies, etc., which has V-shaped bottom cutting edges each having an inclination angle (center-recessed, inclined angle) of 4° or more in nose portions of the ball-end cutting edges. However, because ball-end cutting edges and peripheral cutting edges have small twist angles, chipping and breakage cannot be sufficiently prevented in high-feed roughing of a high-hardness, difficult-to-cut material.
JP 2010-105093 A discloses an end mill comprising ball-end cutting edges each having a rake angle of −10° to 0° (0° or negative near an outer end), and peripheral cutting edges each having a positive rake angle, a rake face of each ball-end cutting edge entering a rake face of each peripheral cutting edge. However, because this ball end mill does not have center-lowered, inclined cutting edges, the clogging of chips occurs near the rotation center point in high-feed roughing of a high-hardness, difficult-to-cut material. Also, because the ball-end cutting edges and the peripheral cutting edges have small twist angles, chipping and breakage cannot be sufficiently prevented in high-feed roughing of a high-hardness, difficult-to-cut material.
JP 2006-15419 A discloses a ball end mill having substantially quarter-circular bottom cutting edges (ball-end cutting edges) and peripheral cutting edges, which have substantially the same radial rake angle in their connection points. JP 2006-15419 A describes that with such a shape, the cutting edges have strength not largely variable in their connection points, thereby avoiding cracking and chipping due to the concentration of a cutting load even when the ball end mill is used for cutting to their connection points. However, JP 2006-15419 A does not consider the radial rake angles and twist angles of ball-end cutting edges and peripheral cutting edges. Also, because a rake face of each peripheral cutting edge largely enters a rake face of each ball-end cutting edge, with each ball-end cutting edge not having a convex rake face, each ball-end cutting edge of this ball end mill has insufficient rigidity and strength. Accordingly, when this multi-flute ball end mill is used for high-feed roughing of a high-hardness, difficult-to-cut material, chipping and breakage occur in the ball-end cutting edges and peripheral cutting edges.